Abstract: Rhythmic beating of the heart is controlled by electrical impulses initiated by sinoatrial (SA) node pacemaker cells (PCs). SA node dysfunction manifests across a broad range of human cardiac disease and is currently the leading cause for the surgical implantation of mechanical pacing devices. Regardless etiology or age of presentation, the cellular defects that trigger SA node dysfunction are poorly understood, highlighting the urgent need to define the cellular, molecular, and microenvironmental interactions that support and sustain PCs electrical activity. Of significant interest to this proposal, PCs have the unique capacity to rhythmically initiate electrical impulse under ionic conditions that should theoretically suppress their activity. It is becoming increasingly apparent that specific cytoarchitectural features including the lack of high-conductance intercalated disks and small cell size, confer electrogenic characteristics that protect PCs from ionic suppression. Dysregulation of PC cytoarchitecture, therefore, represents a significant vulnerability to electrical dysfunction and cardiac arrhythmia. Currently, almost nothing is known regarding the regulation and/or maintenance of PC cytoarchitecture. The long-term objectives of this proposal are to address this fundamental gap in current knowledge by defining the developmental events that initially pattern the phenotypic features required for PC function. Our overall working hypothesis is that unique microenvironmental conditions present within the forming SA node suppress adherens junction formation which, in turn, promotes the cellular attributes that support PC excitability (i.e. small size and poor electrical coupling). This hypothesis will be tested in three specific aims that will define whether the SA node microenvironment controls cytoarchitecture (Aim 1), establish whether loss of adherens junction formation regulates PC size/electrical activity (Aim 2), and identify the upstream molecular regulators of the PC phenotype (Aim 3). By defining the events that pattern PC cytoarchitecture this proposal will create a new comprehensive and mechanistic model of PC development. Furthermore, by defining the conditions that pattern and maintain PC phenotype, these studies will uncover pathways that may become disrupted in juvenile and/or adult cases of SA node dysfunction, as well as establish basic cell biological paradigms that will need to be accounted for as cellular-based therapeutics for the correction of cardiac arrhythmias continue to advance.